import java.util.HashSet;

public class LinkCycle {
    static class  ListNode {
        int val;
        ListNode next;

        public ListNode(int val, ListNode next) {
            this.val = val;
            this.next = next;
        }
    }

    public static void main(String[] args) {
        ListNode node5 = new ListNode(5,null);
        ListNode node4 = new ListNode(4,node5);
        ListNode node3 = new ListNode(3,node4);
        ListNode node2 = new ListNode(2,node3);
        ListNode node1 = new ListNode(1,node2);
        node5.next=node3;

        System.out.println(hasCycle(node1));
        System.out.println(hasCycle2(node1));
    }
    //双指针
    private static boolean hasCycle2(ListNode head) {
        if(head == null||head.next==null){
            return false;
        }
        ListNode slow = head,quick = head.next;
        while(slow!=quick){
            if(quick.next==null||quick== null){
                return false;
            }
            slow = slow.next;
            quick = quick.next.next;
        }

        return true;
    }

    // 标记法
    private static boolean hasCycle(ListNode head) {
        HashSet hs = new HashSet();
        while(head!=null){
            if(!hs.add(head)){
                return true;
            }
            head=head.next;
        }

        return false;
    }

}
